1. Field of the Invention
The present invention relates to an NOx purifying catalyst that removes NOx in exhaust gas with sufficient efficiency from, for example, automobiles, and in particular relates to an NOx purifying catalyst that removes NOx with sufficient efficiency even under low temperature operation such as diesel engine automobiles.
2. Related Art
An NOx purifying catalyst that is capable of reducing NOx in the exhaust gas discharged from automobiles and the like efficiently has been investigated. Most importantly, in diesel engine automobiles, the reduction of NOx has been a major challenge, as well as the reduction of PM (particulate material).
Generally, in diesel engine automobiles, an oxidation catalyst (DOC) and a diesel particulate filter (DPF) are provided in the exhaust path of the diesel engine. However, a combination of the conventional DOC and DPF are insufficient if a further reduction of NOx is required in the future.
An example of the process for further reducing NOx in the exhaust gas includes an exhaust purifying device for internal combustion engines using an NOx absorbent disclosed in Japanese Patent No. 2600492. The NOx absorbent absorbs NOx while the air-fuel ratio of the exhaust gas is lean and release the absorbed NOx when the oxygen concentration of the exhaust gas is lowered. Therefore the exhaust purifying device releases the NOx absorbed while the air-fuel ratio of the exhaust gas is lean from the NOx absorbent when the oxygen concentration of the exhaust gas is lowered.
However, alkali metals, alkali earth metals and the like are used in NOx absorbents employed in the exhaust purifying device as disclosed in Japanese Patent No. 2600492. In addition, a noble metal such as platinum is used for the oxidation of CO, HC, and NOx under lean conditions and for the reduction of NOx under rich conditions. According to Japanese Patent No. 2600492, the use of strongly basic materials as the NOx absorbent of the device causes a problem in that the noble metal is readily deactivated and NOx purifying performance is deteriorated, particularly in low temperature operation.
In addition, the use of the strongly basic materials as an NOx absorbent makes binding with SOx, which has higher acidity than NOx, stronger. Therefore, a problem arises in that regeneration after sulfur-poisoning requires a higher temperature, which makes low temperature catalyst regeneration more difficult.
In other words, the device according to Japanese Patent No. 2600492 produces the desired effect when applied to gasoline engines operated primarily under high temperature conditions; however, its NOx reduction effect is insufficient when applied to diesel engines and the like operated primarily, for example, 300° C. or less.